In bowl of stand mixer or mixing bowl with electric mixture, combine sweetened condensed milk and cold water, then add pudding mix and beat/blend for 2 minutes. Transfer to bowl, cover and refrigerate for at least 4 hours.
Beat heavy cream in bowl of stand mixer until fluffy and medium-stiff peaks form. Fold whipped cream into chilled pudding mixture and fold and stir until just combined and there are no streaks.
Assemble banana pudding. Peel and slice bananas. In a trifle dish, or any large dish or pan, add a layer of pudding, a layer of wafers, and then a layer of banana slices. Repeat with two more layers, ending with the pudding mixture on top. Amount of layers depends on size of dish.
Cover and chill for 6 hours to overnight. Garnish with crushed wafers and banana slices, if desired.
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Notes
You can go as plain or fancy as you'd like. Use your best trifle dish or layer it in a cake pan. Even use a plastic bowl with lid for easy transportation.